Retrofit lamps having light-emitting diodes are known that can be fitted into existing luminaires as a replacement for the fluorescent lamps originally provided. For the known retrofit lamps it may, however, be necessary to modify the existing luminaire because said retrofit lamps cannot be operated on a specific existing conventional ballast for fluorescent lamps but are designed only for operation from an ac supply voltage. Known from US2008/0094837A1 is a retrofit lamp of such kind which, while being compatible with a conventional fluorescent lamp both mechanically and with regard to its holder, cannot be operated directly on a conventional ballast having a starter and not at all on an electronic operating device. For the retrofit lamp to be properly operated it is necessary to remove the original starter and bridge it with a jumper.
Retrofit lamps that can be fitted directly into existing systems as a replacement for fluorescent lamps are likewise known. An existing system of such kind for fluorescent lamps having a conventional ballast is shown in FIG. 1. The ac supply voltage UN is ducted via a choke D and applied to the fluorescent lamp at one side each of lamp electrode 1-2, 2-2. The other sides of lamp electrodes 1-1, 2-1 are connected to a starter S. For operation in systems of such kind it is necessary only to replace starter S with a jumper or fuse Si and to replace fluorescent lamp LL with retrofit lamp 5 as shown in FIG. 2. A direct connection of the two pins 2-1 and 2-2 is provided for shock-hazard protection reasons on a first end of the retrofit lamp so that the alternating voltage being applied to pin 2-2 will be looped through onto pin 1-1 via fuse Si. The operating device that operates the light-emitting diodes provided as light sources in the retrofit lamp is connected to pins 1-1 and 1-2 on the second end. Operating safety will be ensured when the retrofit lamp is being fitted because pins 2-1, 2-2 of the retrofit lamp's first end are galvanically isolated from pins 1-1, 1-2 of the retrofit lamp's second end so that someone putting one end of the lamp into its holder and touching one of the pins at the other end cannot receive an electric shock.
Known also as conventional ballasts especially in NAFTA countries are what are termed rapid-start circuits. FIG. 3 shows a conventional ballast 7 of such kind consisting substantially of a transformer with two windings each having a tap. The two windings are connected to each other at one end. The respective other winding end and the associated tap are connected to an electrode of fluorescent lamp LL. The ac supply voltage UN is applied to the connection between the two windings and to a tap. Were a retrofit lamp to be designed for a circuit of such kind, then the retrofit lamp's internal electronics unit would need to evenly load the two electrodes or pins on the ends because conventional ballast 7 would otherwise be subjected to an asymmetric load. A known circuit for a retrofit lamp is shown in FIG. 4. One ‘electrode’ of the retrofit lamp is connected to a full-wave rectifier GL1 and the other to full-wave rectifier GL2. The two rectifiers are connected in parallel on the direct-voltage side and connected to the direct-voltage side is an operating device 22 that operates light-emitting diodes 55. Because the direct-voltage outputs of the two full-wave rectifiers GL1 and GL2 are connected in parallel there is a galvanic coupling between the retrofit lamp's two ends. Someone wishing to fit a lamp of such kind into its holder will be at risk because if only one end of the retrofit lamp is inserted (fitting the lamp at an angle), the other end can conduct in certain fault situations and can also be touched. While normally an operator who touches the retrofit lamp's contacts will be protected by the full-wave rectifiers' blocking diodes, a fault may cause one of the diodes to fail, for example, so that a current can flow from one side to the other and through the person who is touching. That poses a substantial hazard for the operator, which is why circuits of such kind are generally not allowed. The retrofit lamps shown in FIG. 2 exhibiting galvanic separation cannot be used owing to the asymmetric loading of the conventional ballast in the rapid-start circuit.